1. Field of the Invention
The present invention relates to an imaging apparatus and a focusing control method. More particularly, the present invention relates to an imaging apparatus, e.g., an electronic still camera and a video camera, and a focusing control method utilized for the imaging apparatus.
2. Description of the Related Art
A conventional method to focus on an object in an imaging apparatus, such as an electronic still camera, includes an autofocus system which automatically moves the position of a focus lens using a luminance signal obtained from an image sensor, such as a charge-coupled device (CCD), so as to perform a focusing operation. A general automatic focusing apparatus using the autofocus system calculates an in-focus point by detecting a position of a focus lens enabling the highest contrast based on a focus evaluation value obtained by integrating high-frequency components of a signal in a focus adjustment area set in a pixel area of the image sensor.
However, the automatic focusing apparatus has problems, when illuminance of an object is low or the amount of a high-frequency component is small due to low contrast of an object, that a focus evaluation value becomes low. Further, the ratio of noise components included in a luminance signal becomes high, so that focusing accuracy decreases.
In order to solve the above-described problem, Japanese Patent Application Laid-Open No. 11-215426 discusses an autofocus apparatus which expands a focus adjustment area when the illuminance of an object is lower than a predetermined value, thus improving signal to noise (S/N) ratio and improving focusing accuracy at low illuminance.
Japanese Patent Application Laid-Open No. 2000-307932 discusses an autofocus apparatus which acquires focus evaluation values in a plurality of focus adjustment areas and performs the following control when a peak of a sufficient focus evaluation value is not obtained in each focus adjustment area. That is, if there is a plurality of focus adjustment areas having similar peak positions of focus evaluation values, the autofocus apparatus selects these focus adjustment areas, calculates an average position of the peak positions in respective focus adjustment areas, and sets the calculated average position as an in-focus position. Thus, the autofocus apparatus can improve reliability of a focus evaluation value peak at low illuminance.
However, when the autofocus apparatus expands a focus adjustment area as discussed in Japanese Patent Application Laid-Open No. 11-215426, an object other than a main object, e.g., a background, may enter the focus adjustment area, and thus the apparatus may focus on an object other than a main object.
Furthermore, when a focus adjustment area having a similar peak position of selected focus evaluation value is a focus adjustment area including an object other than a main object, as discussed in Japanese Patent Application Laid-Open No. 2000-307392, the apparatus cannot select a frame including the main object and, thus, cannot focus on the main object. Further, when an average position of peak positions in a plurality of focus adjustment areas is calculated, the apparatus cannot cancel randomly generated noises and, thus, cannot improve S/N ratio.